This invention relates to a device for forming a two-tiered helical gear, comprising upper and lower parts having different diameters, by sintering a powdery material. One type of the device is for forming external gears and another type is for forming internal gears.
Conventional devices for forming two-tiered sintered helical gears are disclosed in Japanese Patent Publication 48-29900 and in U.S. Pat. No. 3,694,127 corresponding in part thereto. The devices comprise a die, a core, an upper punch and a lower punch concentric with the upper punch and having a regular cylindrical shape. The lower punch is divided into inner and outer pieces. One of the disclosed devices, for forming two-tiered external helical gears, is provided with helical teeth on the outer periphery of both the inner piece of the lower punch and the upper punch, on the inner and outer peripheries of the outer piece of the lower punch, and on the inner periphery of the die.
The other disclosed device, for forming two-tiered internal helical gears, is provided with helical teeth on the inner and outer peripheries of the inner piece of the lower punch, on the inner periphery of both the outer piece of the lower punch and the upper punch, and on the outer periphery of the core. In either device, in order to form gears in a stepped manner, the outer piece of the lower punch and the inner piece of the lower punch have different heights of their top ends.
In such a conventional device, because the lower punch is divided into inner and outer pieces its structure tends to be complicated.
Also, if it is desired to form a gear having a small difference in diameter between its upper and lower parts, the outer piece of the lower punch has to be made thin. Such a punch is liable to break. If the required diameter difference is smaller than a certain level, it may become impossible to provide a two piece lower punch.
Further, a wedge and a fork may be sometimes necessary to pull down the outer piece of the lower punch from the point where the pressing has ended and thus to knock the compacted article out of the die. This will further complicate the structure of the device itself.